More than 60 years on, the case of Helen Duncan, the last woman in Britain to be jailed for witchcraft, refuses to die. As her supporters seek a posthumous pardon, evidence has emerged that she may have been the victim of a plot involving British intelligence agents, including Ian Fleming, creator of James Bond.

In the 1940s Duncan, a Dundee housewife and mother of six, travelled the country performing seances for a war-weary public often seeking reassurance about their loved ones. As a 'materialisation medium', which involved her going into a trance and producing 'ectoplasm' through which spirits would take on earthly features to communicate with the living, Duncan built a reputation as one of spiritualism's greatest heroines.

However, during a sitting in Portsmouth on 19 January, 1944, Duncan, 47, fell foul of the security services when a sailor from HMS Barham is alleged to have formed in ectoplasm and greeted his surprised mother sitting in the audience. His death had been kept a secret by the Admiralty, which had been trying to conceal news of the ship's sinking three months earlier.

Fears that Duncan had access to secret information alerted the security services, and an investigation led to her trial at the Old Bailey, accused of contravening the Witchcraft Act of 1735 by pretending to 'bring about the appearances of the spirits of deceased persons'. She was jailed for nine months.

At a time when the military authorities were anxious to keep plans of the Allied invasion of occupied France secret, Duncan and other psychics were seen as a potential threat to security. Drawing on new research and trial documents released to the National Archive, an academic and award-winning film-maker, Robert Hartley, has claimed that the evidence points to a state conspiracy to crack down on security leaks ahead of D-Day by making an example of Duncan.

'In the run-up to D-Day, the authorities were paranoid about potential security leaks and Duncan was in danger of disclosing military secrets during her seances,' said Hartley. 'Helen Duncan was giving out very accurate information. There were other mediums round the country giving out news on soldiers that had died and someone in authority took it seriously, whatever the source of the information. D-Day was coming up and it was absolutely essential to keep the Allied deception plans intact.'

After examining all the documents, Hartley believes there is evidence to suggest that Duncan's conviction by an Old Bailey jury in March 1944 was unsafe. In a new book, Helen Duncan: The Mystery Show Trial, he suggests that among those responsible for the conspiracy to convict Duncan was Fleming, a key figure in the naval intelligence services, and John Maude, the prosecuting counsel at the trial. 'I am convinced naval intelligence were working with MI5, and when I began looking at that connection Ian Fleming's name kept cropping up as being involved with people either involved in the case or on the sidelines,' said Hartley.

More than half a century later, Duncan's case remains a cause celebre, with more than 30,000 websites, translated into several languages, detailing her story. The 'official Helen Duncan website' claims to have received at least 42 million visitors in the last few years, leading to a worldwide campaign for justice and a petition to the government calling for the dowdy woman, who died in 1956 and is now regarded as a spiritualist martyr, to be pardoned.

Despite popular belief, Helen Duncan was not the last person to be prosecuted in Britain for witchcraft. In September 1944, after the D-Day invasion, Jane York, 72, from Forest Gate, east London, was charged with seven counts of pretending to conjure up spirits of the dead. She was bound over for the sum of £5 to be of good behaviour for three years. Duncan's comparatively heavy sentence just months earlier has been cited as further evidence that she was being made an example of. 'It seems clear to me that the security services conspired to imprison Helen Duncan as part of the tight security operation undertaken in the run-up to D-Day,' said Hartley. 'It was the Admiralty's view that she posed a security risk that needed to be dealt with.

'I appreciate that the conspiracy was undertaken with the intention to protect the lives of allied servicemen and women but now, over 60 years later, it is time to put right this wrong, otherwise it continues to undermine the very rights our nation was fighting for.'